Wattman is total crap - only 2 parameters are available and they do not affect on mining at all
I tend to agree with this statement.
However, I finally had a marginal success with it last night. I set the memory voltage down to 0.870mV and bumped the frequency up to 2150MHz. Dropping the voltage gave me some margin on power, and the increased memory throughput raised the hash rate from 24.5 MH/s up to 25.8 - 26.3 MH/s. A little victory, I'll take it.
Wattman is just a huge power equation. If you drop the voltage, it just tries to raise the core clock rate up to utilize the extra margin. Hashing needs increased memory bandwidth, though, not core bandwidth. Ethash is memory-bound.
I would love to try out the Polaris Editor and tweak things manually, rather than have Wattman figure things out, but I'm almost positive that means moving over to Linux because the ROM won't be signed.
If anyone could confirm or refute this last claim, I'd be happy either way!